Hello Pete, 

Thanks for reaching out with your question. 

Your old treated wood may actually be more likely to contain harmful chemicals than newer treated wood.  I've attached link from the University of Maryland Extension below. Here is an excerpt.

• Micronized Copper Azole (MCA) is currently the most widely available type of wood preservative for residential use. It does not contain arsenic, but it does contain copper. It is considered to be low in toxicity. For information on the health risks associated with MCA-treated wood, refer to the National Pesticide Information Center Wood Preservatives page.
• An older type of wood preservative called Chromated Copper Arsenate (CCA) contained arsenic, copper, and chromium. CCA-treated wood is no longer available for residential use as of 2004.
• Avoid using older types of wood treatments such as CCA, creosote, and Penta-treated lumber.
• Research from Oregon State University showed that pressure-treated lumber does increase soil copper concentrations by a minor amount, but only within 1 inch of the raised-bed edge. Their study found no increase in copper concentration in the plants that were grown in raised beds made with pressure-treated lumber.
• If you have concerns about using MCA-treated wood for raised beds, paint or stain the wood, use a heavy plastic liner between the wood and your soil, or use a non-wood building material (e.g., stones, bricks). There are Additional Resources below.

https://extension.umd.edu/resource/safety-materials-used-building-raised-beds/